Great thread!

One thing about the FAA and US manufacturers...

NickLappos said
Isn't a U.S. manufacturer together with FAA responsible for introducing us to the term "LTE" to cover a design-compromise, or am I completely wrong? -That happens alot :O
I know that Nick is partially referring to the military but everything flying on the civilian market is a compromise, and those are beeing made by every manufacturer, no matter where they're based.

Well, I'm learning alot anyway.

Cheers!
/2beers

If servo transparency and jack stall are being used interchangeably and incorrectly, could somebody explain the difference?

As to servo transparency in the 350/355-
In more foolish days, I'd fly these like ordinary state-side helos.
Flown vigorously- Say, at better than cruise speed- load a little "G," like a zoom and control RPM with collective, add a turn- and the normally impressively agile aircraft's cyclic will stop moving in one direction( or you'll get a "limit" light in a 355). A little less turn (left, last time), or a little less collective and you get some back. Load it again, even with less pitch- and there's the wall, once more. It ruins your plans for hotting up the dog. If there's something that you were turning or zooming to avoid, well- too bad- you're not going to push the stick in that direction, right now. I hope it's a forgiving obstruction, or you left an "out."

I've never encountered this in any other mode besides playing around. And never in any other helo.

The terms can confuse you. We call it servo stalling, or jack stalling but "'Transparency" is a good ephimistic way to describe the same thing - the servo reaches its maximum force and then passes the forces to the crew (becomes "transparent").

The idea that this is used to protect the rotor components from flight loads in certainly novel! We always thought it was a good idea to make them strong enough to not bend!

Frankly, the FAA "Proof and Operations" tests on the helicopter controls forces the rotors to be strong enough.

Regarding LTE as an example of the pot and kettle calling each other black, I don't think you would find me supporting that either, much to Sultan's ire!

I wish I had had the benefit of this thread some years ago, cos it would have saved me putting some embarasing photos in my flying album.

Having sadly been on the receipt of "jack Stall" in a French made heli, there is one aspect to this topic that has not been considered. If you are taught that if you place the aircraft in a 160kt dive and pull back harshly on the cyclic it will 'js'. Why bother with all that when it does the same thing at 30deg AOB/5deg nose up and 60kts in a RH decending turn.

If I could offer any advice it would be that the 'envelope' is something of a vague line that exists in the grey areas only probed by the BOI. Dont be under the illusion that your handling has to be harsh for the aircraft to Jack stall, and the feeling through the sticks can be minimal when you are in full blown 'js'.

All the above is naturally 'IMHO'

Since the question was if any other aircraft have this kind of behaivor, yes sort of the same, the Sikorsky S-55T (sorry Nick). It might not really be a jack stall I think, more like a very veak servo system to very heavy blades, or it might be same thing but just feel diffrent to the pilot.

The S-55T is VERY easy to get in to a bladestall situation if not adhering to the VNE table and follow the WAT reduction closely, especially in mountain flying with gusts. But before it bladestalls on you it starts letting control forces through the servos to warn you that you are getting close. And on the S-55 you can't miss if you loose the servos.

I have never had the questionable "honour" to experience a blade stall in any helicopter, and I hope I never will, but I have for sure had some frightning cases of control forces through the servos on the S-55. I know of at least 3 cases of bladestall on the S-55 at our company in the past, and all went well.

The T version is flying with roughly 20 rpm higher MR rpm than with piston engine (199 to 219), so i wonder how bad the original was. But I guess it did not have the power to reach VNE with max gross as the T version does. In the RFM on the original S-55 it is actually a great deal of text regarding how to avoid and how to cope with bladestall.

Good ol days ;)


CB:{

quote

Nick's post certainly convinced me of the advantages of riding Igor Iron.

Did you really need that post to be convinced ?

I know of some people who would have been more than happy to ride a jackass Star when a blade of the Igor Iron they were on decided to part.

Don't want to start another Airbus vs Boeing or Bell / SK vs EC, though.

Cheers

ATN

ATN,

If you are referring to the Bristow S-76 that shed the blade after it was put back into service after being struck by lightning....I would suggest one might look towards the operator that trashed almost every moving part on the Lightning struck aircraft and after having done that for safety sakes....then sent the blades off for inspection and then put it back into service. The 76 had two other blade failures in its very earliest hours....and knew one of the guys killed in the second accident (a man I respected highly in all regards I might add.)

The same company lost Bell aircraft to blade issues, Wessex aircraft to unknown causes....but these are all catastrophic failures.

My point is I have concerns about knowingly flying a machine that in "normal" flight can have control problems. That does give reason for pause or should anyway. Logically, it does not follow that one would wish to fly a helicopter whose controls "lock" up in the cruise....or while maneuvering near the ground.

Having only flown in the back & front of helicopters for nearly 50 years, I seem to have missed out on all these jacking problems you guys seem to have had.

The closest to it I can recall is in a Bell 47 & 206 when flying in mountainous areas parallel to a cliff face in very windy conditions, maybe 30 + kts, a gust has hit me towards the cliff face and I could not move the cyclic in the opposite direction. The cure was to move the cyclic towards the cliff to unjam the cyclic and then put in opposite cyclic. Is this what you mean by jack stall? I just considered it as running out of hydraulic control.

I can't imagine this in normal cruise unless you are flying through a massive thunderstorm when anything can happen.

ATN,
I certainly don't want this to become an us and them thing. I believe that the servo strength of virtually every helicopter I know, civil and military, does not show any jack stall in regions anywhere close to the operating envelope. This 350 discussion is teaching me plenty. I do believe that many/most/all of the helicopter world (British, US, Russian, etc) has grown past the need for the pilot to worry about having his helo take control away from him in most maneuvers.

Collective Bias, that S-55 condition you mention is most certainly stall related servo load, but happens at or beyond Vne, and to an aircraft that is arguably three generations old. If jack stall was only experienced at Vne in 1949 (S-55 first flight Nov, 1949!), one would have hoped we stayed the same or got better in 55 years!

I did the servo adequacy tests on the S-76, and took a loaded S-76 to Vne, turned off one servo system, then maneuvered to 1.7 g's to clear the aircraft for certification. Not a hint of jack stall, believe me.

The hydraulic belt on the AS350 has caused some problems. One was lost in the GOM a few years back when the belt broke and the pilot tried to land on an offshore platform. Sadly, there were no survivors. I have around a thousand hours in the AS350D, and always had mixed feelings - lots of fuel, plenty of speed and comfort, but if things went wrong, they went badly wrong.

Two tales, but both concerning SA/AS/EC type helicopters and servo/hydraulics:

1. SA-341 Gazelle (MIL version) suffered a fatal accident at Farnborough around 1973. The helicopter performed a low level, right-hand turn at high speed. The roll to the right continued unchecked and the helicopter crashed inverted. The conclusion was that by increasing the angle of attack on the advancing blade, which provides most of the lift, the forces on the blade at high forward speed were sufficient to overcome the hydraulic servo and jack stall occurred, and the pilot was unable to counter the roll.

This occurs mainly when turning to the right - French helicopter = advancing blade on the left side of the helicopter. When turning left the angle of attack on the advancing blade decreases, and the forces on the servo decrease accordingly.

Even though this was a known phenomenon, and demonstrated under training, it was the presumed cause of another Gazelle fatal accident in 1975, during a low level sortie over Dartmoor, killing the instructor and student.

2. AS-350B2 – When this type was introduced it had new spherical bearings, new single hydraulic system (both taken from the 355 dual hydraulic system), a new power plant, beefed up rotorhead, & etc. Better, more powerful version of the B & B1……?

The hydraulic pump is driven by a belt (“rubber band” - see previous postings on this). In cold weather (under - 25oC) the hydraulic fluid increases in viscosity and the belt starts to slip. As it cannot drive the pump at normal speeds, the belt deforms, due to the friction from the accessory drive, and eventually stops turning the hydraulic pump, or slips off the drive wheel. OK you think, small helicopter, no hydraulics, what’s the problem ?
Well it turns out that the new spherical bearings (“rubber balls” from the 355) freeze solid at temperatures under - 25oC, and without a hydraulic servo the whole control system locks up !

The pilot then becomes a passenger in the helicopter, and only by using extreme force can the controls be moved.

AS/Eurocopter say they have fixed this problem with new types of spherical bearings, but the fact remains that if you lose the hydraulic drive belt on a 350, it becomes extremely difficult to control – see the news gathering AS-350 that plunged to a rooftop in New York earlier this year

Reading Collective Bias's story about the S55T reminds me of an incident I had some years ago when a leak in the cockpit roof caused a drip resulting in a bridging of the hydraulics Primary off, Secondary off test switch.

Both sets of hydraulics failed on me leaving only the emergency system - driven I think by transmission oil pressure on the lateral jacks. All could have been restored by switching off the electrical master switch (fail safe on) but I lacked the courage and free hand to reach for the switch.

From that incident on I made a special effort to learn the aircraft systems and not just skim over the text to pass the type rating exams.

Yes I did manage to get the aircraft down and land it after 20 minutes flying in that condition - pure luck.

Jim

Reading about the accident involving a hard right turn near the ground....and being unable to stop the roll and for sure probably not wanting to reduce collective at that point....a question arises.

The BO-105 and BK-117 have a similar trait in that situation....one can run out of cyclic authority if a high roll rate occurs.....the recovery is to apply "full" opposite pedal.....fully and quickly...."stomp" was the word used but something slightly short of that seemed to work during demos.

Adequate control is quickly gained and as the nose pitches up to level or more....then the collective can be lowered and cyclic authority is regained.

Just a thought!

This is a very intresting subject and I have to send my 2 cents again.


BB I think the problem you refer to was from an accident on Greenland in the mid or late 80 ies. This was due to (if I remember right) a broken drive belt to the hydraulic pump in very cold conditions and with a new style of Spherical Bearing (load carrying rubber bearing) fitted. Not to the fact of slipping drive belt.

The fix was a new style of bearing, and a new lower temp limit of the previous one.

The RFM also calls out for warming up the spherical bearings before takeoff in cold conditions by moving the cyclic 3-4 cm fwd for 2 min and to check the force to move the cyclic without servo.

We have been flying B, B1, B2, B3 in very cold conditions (down to lower limit) for a long time without problems, we started with B in 1979. We even tested a cog belt driven pump for EC, but it never went into production. Even if the belt drive looks very simple, it has just failed on us once in all these years, and it was a successful landing afterwards.

I have always as a training captain had the opinion on single hyd system helicopters that if there is a procedure in the emergency checklist that calls for turning off the hyd system ASAP, then it should also be trained at cruise speed. Normal procedure is to bring down the speed to around 60 KIAS and then turn it off (even at ECF they do not train this at high speed if they have not changed there way of training). But if you get some sort of hardover you need to get it off immidiately, no matter the airspeed, without having tried it before it could be intresting

:ooh:

Therefore when I train someone on the 350 I demonstrate this and then let the pilot try it with increasing airspeed up to cruise. At cruise it is hard work, but since the cyclic want to go right and aft, it is very much controllable. But since many 350 pilots from time to time hold the cyclic "very light" to get good feel, I think it is important to know what they are up against if you turn off the servo at cruise speed.

:ok:

CB

I taught on AS 350Bs for a few years. One of the course demonstrations was"jack stall". The point of this demo was to show the trainee that the phenomenon was possible and could be dangerous close to the ground.

It was a while ago but there was very rapid rolling and pulling to achieve it. Once it occured, it felt much like the aircraft did hydraulics off. Although the controls were heavier they could still be moved and the aircraft recovered. You really had to work hard to make it happen.

It's a limitation to operations, like left crosswinds and LTE are to Bell 206s, left pedal stops and crosswinds to Bell 205s, vortex ring is to any helicopter and the size of the fuel tank is to any aircraft. No big deal.

As for the "rubber band", the fleet of about 20 to my knowledge never had a failure, even in a temperature range from -5 deg C to 40 deg C. The biggest problem with them was getting them on and off. Undoing drive train etc. They were lifed for 10 years so when the fleet hit 10 years old, not enough rubber bands to go round.

I've seen a few cases of sudden belt slippage when arriving in the hover in loose dry snow (read - - big nasty white snowball ! )

This not the time you need control issues, horns, etc as you are searching for the ground in a white-out ! I have also seen a few broken belts, plus a lot of burned - glazed ones.

I work with a 100% Bell company now( :ok: ), but my past employer used to have a spare belt fastened around the drive-shaft of each 350, so the complete dis-assembly of the drive system was not required to change the belt -- once anyway.

I think a lot of the hydraulic problems mentioned above during high speed turns are not really hydraulic problems but retreating blade stall which causes a euro machine to flick right and a US machine to the left. For this reason when teaching steep turns, you get the student to turn the opposite way to the flick, so that if it goes wrong, you flick upright.
I well remember being number 5 in a tail chase in a S55 when the leader sped up close to vne and wrapped on lots of right bank. A few seconds later, trying to follow number 4, I was doing a steep turn to the left. Very worrying but not really a hydraulic problem.
Maybe the Gazelle had this happen? :confused:

The Gazelle's controls give a definite feedback "jolt" when jackstall occurs. I saw retreating blade stall in a Whirlwind 10 (at 150 feet agl) and if I recall correctly, it didn't give the same effect.

Both have a similar effect on the pilot though...... :ooh:

407 Driver's Comment
 

This is a very good example of how prone the ASTAR is to jack stall. It doesn't take very much. At high DA's in combination with a High gross weight, light to meduim turbulance can result in jack stall. Anyone who has flow an ASTAR in the mountains has probably experienced this.

Mr Osborne, my experience is contrary tp your speculation regarding blade stall. I've had it happen in turns of both directions, and vigorous but not terribly rapid roll rate or exceptionally high speed- well within the green arc and low DA. The surest way I've induced it is cruise or higher, pull some g, and roll. The stick just stops, lateral. I'd hate to force it, as the onset is sudden and unpredicted- it could well cease with me leaning on it- That would introduce new and exciting issues. Pitch reduction (both senses of the word), slow, and the aircraft's normal again.

As to belts- An Arizona 350 had an air conditioner compressor drive belt break and take out the hydraulic drive belt, too. It's a chintzy system in an otherwise very satisfactory helo.